a) Field of the invention
The present invention relates generally to a technique for guaranteeing a quality of YAG laser welding on working material pieces by means of a laser welding, for example, a YAG laser in an assembly line such as a vehicular body assembly line.
The present invention, more particularly, relates to method and system for determining a quality of welding at a weld between working material pieces which not only determine whether the welding gives a good result or bad result but also detect variations in parameters such as to give a large influence on the welding quality to identify what is a cause of the bad result (failure) in the welding. The present invention is applicable to a YAG laser welding adaptive control system adopting an adaptive control for the whole YAG laser welding system.
b) Description of the related art
Important parameters such as to be directly related to a welding quality in a YAG laser welding include, for example, a laser output power at a work spot of welding, a position of focal point of the laser (so-called, focal length and which is determined by a beam diameter), a positioning accuracy of the working material pieces (the working materials to be welded together and, hereinafter called, an overlapped seam gap length), a gas flow quantity, and a welding speed.
It is desired that these welding parameters are controlled to fall their values within their predetermined allowable ranges during the welding using the YAG laser. However, during a sequential welding of large-sized working material pieces such as those found in a vehicular body assembly line, it is impossible to avoid completely large variances (or large deviations) in the welding quality for the respectively welds of the working material pieces from a perfect welding quality and large variances (or large deviations) of the welding parameters based on a stopped positional accuracy of each working material piece to be welded by means of a work material piece carrying device. It cannot be said that a sudden, unexpected weld failure does not occur.
A method for measuring a physical quantity such as light or sound which is generated during the laser welding to estimate the welding quality has been proposed as a monitoring technique of the laser welding quality.
For example, a Japanese Patent Application First Publication No. Heisei 10-6051 published on Jan. 13, 1998 exemplifies a method for measuring a frequency distribution of a plasma light emitted during the laser welding using a CO2 (carbon dioxide) laser so as to estimate the welding quality at each weld.
On the other hand, for the YAG laser, an English paper on development items of a German company called Laser Zentrum Hannover e. V. discloses Process Control During Nd: YAG Laser Beam Welding published on June, 1995. In this English paper, a monitoring system has been commercialized which detects an intensity of an emission of the plasma light (it is noted that, since an ionization percentage is low in the YAG laser welding, it is naturally correct to use a term of a plume in place of the term of plasma but it is normally called plasma) and compares the detected waveform with a normal waveform prepared when the good quality of welding has been obtained.
However, in the above-described previously proposed monitoring system disclosed in the English paper, only the intensity of plasma light generated from the weld of the working material pieces is detected to determine the quality of welding and the determination of whether the result of welding at the weld is good or bad is dependent upon whether the detected plasma waveform falls within a range set at a constant percentage to a plasma waveform obtained when the result of welding at the same weld has been determined to be good and stored as a reference waveform.
Hence, even if the good or bad result of the welding quality can be determined, a cause of the bad result of welding (failure in welding) is not yet identified.
Consequently, in order to eliminate the failure of welding, the welding parameters are not yet automatically be controlled according to the previously proposed monitoring method to fall the welding parameters within the allowable limits and a countermeasure to cope with the failed welding is not yet made.
To eliminate such a problem as described above, there is a demand in the YAG laser welding technique to develop the monitoring technique which can also identify the cause of the failure of welding in addition to the determination of whether the result of welding is good or bad.
It is, therefore, an object of the present invention to provide method and apparatus for determining a quality of welding at a weld between working material pieces which can determine a good or bad result of welding and which can identify the cause of the failure when the quality of welding is determined to be the bad result.
As a result of scrutiny on information signal from the weld during the YAG laser welding to solve the above-described problem, it was discovered that when, in addition to the intensity of the plasma light (visible light region) generated from a high-temperature metal vapor developed on the weld (as described above, although the plasma is not accurate in terminology but the plume exactly corresponds to the phenomenon of the metal vapor, hereinafter, called plasma), the intensity of a reflected light of the YAG laser which is reflected from the weld without an absorption of the radiated light on the weld on one of the working piece materials was individually measured and signal levels of both components of a low-frequency component (DC component) equal to or below approximately 100 Hz and a high-frequency component (AC component) which is involved with a large time variation up to about 10 KHz with the DC component intensity as a fundamental frequency component intensity were respectively detected, signal information of a total of four kinds of the information signals, viz., the DC component and AC component of the plasma light emission intensity and those of the YAG laser reflected light intensity indicated their characteristic behaviors to the variations in the welding parameters, for example, the laser.output (output power), the focal point position (defined as a focal length), overlapped seam gap length, and so forth. It was determined that the quality of welding at the weld can indirectly be determined (tendency control) by monitoring the variations in the four kinds of signal information and an accurate estimation from which parameter a cause of failure in welding quality is derived can be made to eliminate the cause of failure in welding.
The above-described object can be achieved by providing a method for determining a quality of welding at a weld between working material pieces, comprising: detecting an emission intensity of a visible light emitted from the weld during a laser welding using a laser device emitting a laser having a wavelength falling in a range of the wavelengths of near infra-red rays; outputting a first detection signal indicating the light emission intensity of the visible light; detecting an intensity of a reflected light of the laser from the weld during the laser welding; outputting a second detection signal indicating the light intensity of the reflected light; analyzing frequencies of the first and second detection signals; and determining whether a result of the laser welding falls in a favorable range of welding and, at the same time, identifying a cause of welding failure of the weld if determining that the result of the laser welding falls out of the favorable range on the basis of signal intensities of a first frequency component of each of the first and second detection signals lower than an arbitrary frequency in a range from 50 Hz to 200 Hz and of a second frequency component of each of the first and second detection signals higher than the arbitrary frequency.
The above-described object can also be achieved by providing an apparatus for determining a quality of welding at a weld between working material pieces, comprising: a first detector to detect an emission intensity of a visible light emitted from the weld during a laser welding using a laser device emitting a laser having a wavelength falling in a range of the wavelengths of near infra-red rays and to output a first detection signal indicating the light emission intensity of the visible light; a second detector to detect an intensity of a reflected light of the laser from the weld during the laser welding and to output a second detection signal indicating the light intensity of the reflected light; and a measuring device to analyze frequencies of the first and second detection signals, to determine whether a result of the laser welding falls in a favorable range of welding, and to identify a cause of welding failure of the weld if determining that the result of the laser welding falls out of the favorable range on the basis of signal intensities of a first frequency component of each of the first and second detection signals lower than an arbitrary frequency in a range from 50 Hz to 200 Hz and of a second frequency component of each of the first and second detection signals higher than the arbitrary frequency.